I'm not sure how accurate this hood would be for our use. Even if you used this for all the vents in a home, it's not going to give you an accurate measurement of the air in the duct itself. Since behind the wall you could have anything from a 5" round duct to a large rectangular return duct. But, if you we're an HVAC pro you would likely have higher-end anemometer that will read the results from the hood and convert it to the correct CFM readings, regardless of duct size.

The trash bag method would be hard pressed to fill accurately(pun intended). The bags would fill so fast that accurate measurements would be difficult. The larger the bag(s) make for easier fill times. Admittedly, flow rates,even with an expensive anemometer, can be difficult if done carelessly. But by taking several readings, or an average reading over several locations within the stream you can have a fairly accurate reading.

Bert-You have enough space to insert a straight duct or use a large bag. Using both would be ideal at least for the initial testing. Once you have an idea of your flow characteristics, you should be fine using only the anemometer.

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